The Proposition 8 case already has a powerful conservative supporter: Theodore B. Olson, the former solicitor general under Mr. Bush and one of the suit’s two lead lawyers. The amicus, or friend-of-the-court, brief is being filed with Mr. Olson’s blessing. It argues, as he does, that same-sex marriage promotes family values by allowing children of gay couples to grow up in two-parent homes, and that it advances conservative values of “limited government and maximizing individual freedom.”

Legal analysts said the brief had the potential to sway conservative justices as much for the prominent names attached to it as for its legal arguments. The list of signers includes a string of Republican officials and influential thinkers — 75 as of Monday evening — who are not ordinarily associated with gay rights advocacy, including some who are speaking out for the first time and others who have changed their previous positions.

“We are trying to say to the court that we are judicial and political conservatives, and it is consistent with our values and philosophy for you to overturn Proposition 8,” said Ken Mehlman, the former chairman of the Republican National Committee, who came out as gay several years ago. He is on the board of the American Foundation for Equal Rights, which brought the California suit, and has spent months in quiet conversations with fellow Republicans to gather signatures for the brief.

In making an expansive argument that same-sex marriage bans are discriminatory, the brief’s signatories are at odds with the House Republican leadership, which has authorized the expenditure of tax dollars to defend the 1996 marriage law. The law defines marriage in the eyes of the federal government as the union of a man and a woman.

House Speaker John Boehner has committed millions of dollars in government funds to defend the Defense Of Marriage Act (DOMA), and the actions of these prominent Republicans could drive a larger wedge into the fissure that separates the sensible moderates in the party, and the extremist fringe.